The present invention relates, in general, to methods of fabricating fluidic devices and to structures produced by such methods, and more particularly to processes including the removal of sacrificial layers for fabricating multi-level fluidic devices integrally with other devices on a substrate for interconnecting such devices.
The emerging field of fluidics has the potential to become one of the most important areas of new research and applications. Advances in genomics, chemistry, medical implant technology, drug discovery, and numerous other fields virtually guarantee that fluidics will have an impact that could rival the electronics revolution.
Many fluidic applications have already been developed. Flow cytometers, cell sorters, pumps, fluid switches, capillary electrophoresis systems, filters, and other structures have been developed using a variety of materials and techniques in a wide range of applications, including protein separation, electrophoresis, mass spectrometry, and others, have been developed. One of the goals of workers in this field is to develop a xe2x80x9ctotal analysis systemxe2x80x9d wherein various structures are integrally formed on a single substrate, and for this purpose a variety of techniques, rag from silicon micromachining to injection molding of plastics, have been developed. All of these prior techniques, however, have in common that fluid capillaries are formed by bonding or lamination of a grooved surface to a cap layer, and in cases where multiple layers are present, these result from bonding together multiple substrates. Unfortunately, the drive to develop complex fluidic devices in the environment of a total analysis system has been hindered by the inherent difficulties with lamination-based fabrication techniques. As more devices are integrated onto a single substrate, the connection of the devices requires that connecting fluidic tubes cross over each other. With bonding technology, two capillaries cannot cross without lamination of a second wafer to the basic substrate. Further, the second wafer must be thick, resulting in large aspect ratio vertical interconnects, and ultimately resulting in a limit on miniaturization. If such devices were to reach mass production, alignment and bonding technology to handle the complex assembly would have to be developed, and whatever technology is employed would in all likelihood require costly redevelopment with each generation of smaller more complex fluidic devices.
One major application of fluidic devices is in the fabrication of artificial gel media, which has been a topic of interest for some years for scientific and practical reasons. Artificial gels differ from conventional polyalcrylamide or agarose gels currently used for DNA separation in that the sieving matrix in an artificial gel can be defined explicitly using nanofabrication, rather than relying on the random arrangement of long-chain polymers in the conventional gel. As such, the dimensions and topology of the artificial gel sieving matrix can be controlled and measured precisely. This makes it possible to test theories of DNA electrophoresis with fewer free variables. Artificial gels also have advantages over conventional gels in that conventional gels are expensive and require skilled operators to prepare them immediately before use, whereas artificial gels can be integrated with mass-produced microfabricated chemical processing chips and shipped in a ready-to-use form.
However, previous methods for fabricating artificial gels involved bonding a top layer, either glass or a pliable elastomeric material, to a silicon die with columnar obstacles micromachined into the surface. Such methods have been successful for structures with fluid gap heights as small as 100 nm, but it is difficult to establish a uniform and predictable fluid gap between a silicon floor and a glass or elastomeric top layer. An elastomer layer, and in many cases even a glass layer, can flow between the retarding obstacles in the fluid gap, either dosing the gap entirely or creating large variations in the gap height. Both methods are sensitive to particulate contamination to the extent that a single particle can render an entire device unusable.
It is, therefore, an object of the invention to provide a method for fabricating multiple fluidic devices as a monolithic unit by the use of a sacrificial layer removal process wherein fluidic devices with one or more layers can be fabricated by successive application and patterned removal of thin films. Some of these films are permanent, and some are sacrificial; that is, they will be removed before the fabrication is complete. When the sacrificial layers are removed, the empty spaces left behind create a xe2x80x9cworking gapxe2x80x9d for the fluidic device which can be virtually any shape, and which can be configured to perform a number of different functions.
Another object of the present invention is to produce nano-fabricated flow channels having interior diameters on the order of 10 nm. Such nanometer-scale dimensions are difficult to attain with conventional micro-fluidic fabrication techniques, but the present invention facilitates fabrication at this scale while at the same time providing integration of such flow channels with other devices. These devices can provide fundamental insights into the flow of fluids in nano-constrictions and are useful in studying the behavior of biological fluids with molecular components similar in size to the cross-section of the channel. The process of the present invention permits the dimensions of the flow channels to be adjusted, for example to manipulate and analyze molecules, viruses, or cells, and the process has the potential of producing structures which will reach currently unexplored areas of physics and biology.
Another object of the invention is to provide a multi-level fluid channels fabricated on a single substrate with fluid overpasses and selective vertical interconnects between levels. Multi-level fabrication is a requirement for any complex fluid circuit, where fluid channels interconnect multiple devices on a single substrate, for without multiple levels, interconnection of large numbers of devices is either impossible or requires tortuous interconnect pathways. Lee available level of sophistication of microfluidic devices is tremendously improved by the capabilities provided by the present invention.
Briefly, the present invention is directed to procedures and techniques for overcoming the inherent difficulties and limitations of prior art laminar bonding approaches to fluidics fabrication and integration of components. In one aspect of the invention, these difficulties are avoided in the fabrication of a monolithic fluidic device by utilizing a shaped sacrificial layer which is sandwiched between permanent floor and ceiling layers, with the shape of the sacrificial layer defining a working gap. When the sacrificial layer is removed, the working gap becomes a fluid channel having the desired configuration. This approach eliminates bonding steps and allows a precise definition of the height, width and shape of interior working spaces, or fluid channels, in the structure of a fluidic device. The sacrificial layer is formed on a substrate, is shaped by a suitable lithographic process, for example, and is covered by a ceiling layer. Thereafter, the sacrificial layer is removed with a wet chemical etch, leaving behind empty spaces between the floor and ceiling layers which form working gaps which may be used as flow channels and chambers for the fluidic device. In such a device, the vertical dimension, or height, of a working gap is determined by the thickness of the sacrificial layer film, which is made with precise chemical vapor deposition (CVD) techniques, and accordingly, this dimension can be very small.
In order to provide access to the sacrificial layer contained in the structure for the etching solution which is used to remove the sacrificial layer, one or more access holes are cut through the ceiling layer, with the wet etch removing the sacrificial layer through these holes. An extremely high etch selectivity is required between the sacrificial layer and the dielectric layers in order to allow the etch to proceed in the sacrificial layer a significant distance laterally from the access holes without consuming the floor and ceiling layers which compose the finished device. One combination of materials that meets the requirements for such a process is polysilicon and silicon nitride, for the sacrificial layer and for the floor and ceiling layers, respectively. Extremely high etch selectivities can be obtained with basic solutions such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), but especially with tetramethyl ammonium hydroxide (TMAH). TMAH provides an etch selectivity between silicon and silicon nitride as high as 1,500,000:1, with etch rates as high as 0.6 xcexcm per minute. Additionally the basic solution contains no metal ions and is thus compatible with the CMOS CVD equipment used to deposit the thin film sacrificial polysilicon layer and the thin film ceiling layer.
The access holes cut in the top layer need to be covered before the device can be used. For this purpose, a sealing layer of silicon dioxide is deposited on top of the ceiling lay to fill in the access holes, and this additional thin film layer provides a good seal against leakage or evaporation of fluids in the working gap. SiO2 CVD techniques which yield a low degree of film conformality, such as very low temperature oxide (VLTO) deposition, form a reliable seal without excessive loss of device area due to clogging near the access holes. If desired, the access holes may be drilled through the bottom layer, instead of or in addition to the holes in the ceiling layer, and later resealed by depositing a layer of silicon dioxide.
In one embodiment of the invention, wherein the process is utilized to fabricate artificial gels, a multiplicity of retarding obstacles in the form of vertical pillars are fabricated in a selected portion of the sacrificial layer before the ceiling layer is applied. The obstacles are defined using standard photolithographic techniques. In another embodiment of the invention, electron beam lithography is used for this purpose, permitting the fabrication of obstacles several times smaller than can be produced utilizing the photolithographic techniques.
In one example, lithography was used to define in the sacrificial layer a filter chamber incorporating an artificial gel and connected to inlet and outlet fluid channels. In this process, an array of holes was formed in a chamber region of the sacrificial layer, the holes being about 100 nm in diameter and separated by 100 nm in a square array, for example. When the ceiling layer was applied, the ceiling material filled the holes to form a multiplicity of pillars about 100 nm in diameter and separated by 100 nm. The pillars extended through the sacrificial material between the floor and ceiling layers, and when the sacrificial layer was removed the pillars formed in the chamber region the vertical obstacles of an artificial gel. The chamber region had an active area 800 xcexcm by 500 xcexcm, with connecting inlet and outlet flow channels, or microchannels, connected to opposite sides of the chamber to make a microfluidic device 15 mm in length. The extra length provided by the inlet and outlet capillaries was provided in the example to allow fluid interconnects to the device to be outside the footprint of an objective lens used to observe material within the filter chamber, but any desired inlet or outlet channel configurations can be used.
The interconnection of the fluid between external devices and the working gap produced by a sacrificial layer, as described above, preferably is made by way of one or more loading windows and outlet windows on the top (ceiling) surface of the inlet and outlet microchannels. These windows are defined with photolithography and are etched through the ceiling layer with RIE. They may be located at the outer ends of the microchannels, which may be near opposite edges of a silicon chip or substrate carrying the artificial gel.
In a typical use of an artificial gel device such as that described above, an aqueous buffer with fluorescent-labeled DNA molecules in solution is supplied to the loading window from a fluid reservoir which forms a meniscus with the edge of the silicon chip, and after passing through the gel the buffer is delivered to a reservoir connected to the outlet window. A potential is applied across the gel by a voltage connected across electrodes immersed in the buffer reservoirs, and the applied potential difference drives the DNA molecules through the device, where their motion is observed with epi-fluorescence microscopy.
In another aspect of the invention, multiple fluidic levels are constructed on a single substrate by repeated applications of the sacrificial layer technique. With this process, barriers between the layers can be extremely thin, because the solid sacrificial layer mechanically stabilizes the film during construction of multiple layer devices. Each layer could potentially add less than 500 nm to the thickness of the device, with miniaturization being limited only by available lithographic or electron beam techniques. The fabrication of multiple-level devices is an extension of the single-level fabrication technique outlined above. The first level is defined exactly as in the single level system, but instead of perforating the ceiling layer to provide access holes for sacrificial layer removal, holes are made in the first level ceiling layer only where there are to be connections to the second level. These vertical interconnect holes are made using the same steps used for making access perforations. If no connections are needed, then no interconnect holes are made in the first level ceiling layer. Thereafter, a second sacrificial layer is deposited over the structure, this layer having a thickness equal to the desired vertical dimension of a working gap in the second layer, and preferably being between 30 nm and 1000 nm in thickness. Photolithography or electron beam lithography is used to pattern the second sacrificial layer to define a desired structure configuration, such as fluid-carrying tubes or microchannels, fluid chambers, or the like. The second level structure may be configured to pass over fluid microchannels that were previously defined in the first-level lithography step, and the first and second level sacrificial layers may make contact with each other where vertical interconnect holes breaching the ceiling of the first level and intersecting the working gap defined by the sacrificial layer in the second level have been provided. Finally, the second level ceiling layer is deposited, in the manner previously described for a single level device, and access holes are defined as before.
If desired, additional layers may be added by depositing additional patterned sacrificial layers on prior ceiling layers in the manner described for the second level, and depositing additional corresponding ceding layers. Thereafter, the sacrificial layers are removed from all layers by a wet chemical etch, as previously described, producing a multiple-level fluidics structure. If all the layers are vertically interconnected through access holes, the sacrificial layers can all be removed together. If they are not so interconnected, the sacrificial layer is removed by way of separate access layers, which may be located at the edges of the multilevel device.
The unique approach to the fabrication of nanofluidic structures in accordance with the present invention offers several advantages over prior processes. First and foremost is the integration of fluidic devices with other devices, such as optical or electronic devices, on a single substrate, without lamination or bonding steps. Such integration can be obtained by reason of the fact that the methods of the invention rely on semiconductor manufacturing techniques and equipment already in existence in the semiconductor manufacturing industry. The ability to create multi-level structures with vertical interconnections allows significant increases in integration and functionality, allowing large-scale integration of fluidic devices and permitting fabrication of structures which allow parallel processing and high speed analysis to be performed. Since the technology is compatible with other fields of microfabrication such as planar waveguide optics and silicon-based microelectronics technology, not only can microfluidic components be integrated with each other, but they also can be integrated on a single wafer with other types of components or devices, such as those required for analysis and data collection. The fabrication techniques of the present invention permit creation of extremely small features with excellent control over dimensions and placement of devices and interconnections so that microfluidic components will be comparable in dimensions to macromolecules, facilitating the fabrication of complex biochemical analysis systems.